Preparation of polymer–rare earth complexes based on Schiff-base-containing salicylic aldehyde groups attached to the polymer and their fluorescence emission properties

2.1 Materials

Styrene (Tianjin Kaixin Chemical Industry Co. Ltd, Tianjin, China) and butyl acrylate (Tianjin Kaixin Chemical Industry Co. Ltd, Tianjin, China) were distilled at reduced pressure. Salicylaldehyde (Tianjin Recovery of Fine Chemical Industry Research Institute, Tianjin, China), hydrazine hydrate (Tianjin Kaixin Chemical Industry Co. Ltd, Tianjin, China), dimethylformamide (Yantai Shuangshuang Chemical Co. Ltd, Yantai, China), tetrahydrofuran (Tianjin Kaixin Chemical Industry Co. Ltd, Tianjin, China), Eu₂O₃/Yb₂O₃ (Beijing Fandechen technology Co. Ltd, Beijing, China) were commercially purchased and used without further purification.

2.2 Preparation of the poly (styrene–co–butyl acrylate) schiff base-containing polymer (PSCBS)

All runs were performed in three stages. Firstly, solution copolymerization of St and BA was carried out under nitrogen atmosphere in a three-necked flask equipped with thermometer and gas inlet tube, and the benzoperoxide (Wt=2%) was added to the copolymer monomer solution, then keep the temperature at 80°C of polymerization reaction for 2.5 h. The poly(styrene–co–butyl acrylate) was obtained after elimination of the residual monomer by washing the crude product with absolute ethyl alcohol and dried at 60°C to a constant weight. (The molar ratio of butyl acrylate and styrene monomers was changed from 1:0.5 to 1:2.5).

Secondly, an ethanolic solution of salicylaldehyde (3.12 g, 25 mmol) was added dropwise to a solution of hydrazine hydrate (1.56 ml, 40 mmol) in ethanol (30 ml) under magnetic stirring at room temperature, and the mixture was kept for 4 h. The solvent was distilled and the pure product was recrystallized from absolute ethanol, and dried under vacuum.

Finally, the mixture of poly (styrene–co–butyl acrylate) and salicylaldehyde hydrazone in tetrahydrofuran were refluxed and magnetically stirred for 24 h. After removing the solvent by vacuum distillation, the residue was further purified by washing with absolute ethyl alcohol to give pale yellow solid product.

Those polymer were abbreviated as PSCBS 1-0.5 (molar ratio as 1:0.5), PSCBS 1-1 (molar ratio as 1:1), PSCBS 1-1.5 (molar ratio as 1:1.5), PSCBS 1-2 (molar ratio as 1:2), PSCBS 1-2.5 (molar ratio as 1:2.5), respectively.

2.3 Preparation of the poly (styrene–co–butyl acrylate) Schiff base polymer –Eu(III)/Yb(III) complexes (PSCBS-Eu(III)/Yb(III))

Eu₂O₃/Yb₂O₃ (20 mmol) was dissolved in 100 mL of HCl solution, heated and stirred for 1 h to make the solution transparent until crystalline grains were produced and amount of crystals formulated by EuCl₃/YbCl₃·6H₂O were spitting out. Then the crystals were placed in a vacuum oven to be dried

The PSCBS samples (1.0 g) were accurately weighed, and placed into five conical flasks of 100 mL capacity followed by adding 40 mL DMF. Under stirring, the PSCBS samples were dissolved, and the DMF solution of EuCl₃/YbCl₃·6H₂O with different concentrations (3.4×10^-3 mol/L; 3.1×10^-3 mol/L; 2.6×10^-3 mol/L; 2.2×10^-3 mol/L; 2.0×10^-3 mol/L) were added dropwise. The coordination reactions between PSCBS and Eu³⁺/Yb³⁺ were placed for 48 h at room temperature and with stirring (the molar ratio of ligand salicylaldehyde hydrazone to Eu³⁺/Yb³⁺ was 3:1). The solution was filtered, and 200 mL distilled water and 1.0 g sodium chloride were added to the filtered with constant stirring. Then, the precipitate was collected by filtration, washed with distilled water and dried under vacuum. These products were transferred into several volumetric flasks (200 mL), and their volume was made constant with DMF by forming six solutions of complex PSCBS–Eu(III)/Yb(III).

Those complexes were abbreviated as C1-0.5 (butyl acrylate and styrene molar ratio as 1:0.5), C1-1 (molar ratio as 1:1), C1-1.5 (molar ratio as 1:1.5), C1-2 (molar ratio as 1:2), C1-2.5 (molar ratio as 1:2.5).

2.4 Physical characterization

Elemental analyses (C, N, and H) were determined with an Elementar Cario EL elemental analyzer. Fourier transform infrared spectra were recorded on KBr disk using Nexus 670 FT-IR spectrometer in 4000~500 cm^-1 region. UV spectra were determined with a UV-2550 Ultraviolet Visible Spectrophotometer. Fluorescence excitation spectra of PSCBS–Eu(III)/Yb(III) were determined using a RF-5301 fluorescence spectrophotometer in the range of 200~900 nm. Differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA) were performed on a STA 449 C TG-DSC simultaneous thermal analyzer with a heating rate of 10 K/min under a nitrogen atmosphere (flow rate: 40 mL/min).

2.5 Determination of spectral performance of complexes

3.1 Synthesis to prepare complexes PSCBS–Eu(III)/Yb(III)

The aromatic acyl hydrazone ligands were bonded onto the side chains of poly (styrene-co-butyl acrylate), and the functional polymer PSCBS was obtained. The molecular weights and molecular weight distributions as well as the glass transition temperatures of the poly (styrene–co–butyl acrylate) were given in Table 1. Finally, the coordination reaction between the macromolecular ligand PSCBS and Eu³⁺/Yb³⁺ ion was carried out in DMF, and the polymer–rare earth complex PSCBS–Eu(III)/Yb(III) was prepared. The preparation procedures and structural characterization methods had been described detailed previously. The Synthesis for preparing the functional polymer PSCBS and the complexes PSCBS–Eu(III)/Yb(III) were illustrated in Scheme 1(1).

Scheme 1

Reaction pathways Schematic expression of synthesis of preparation of complex PSCBS–Eu(III)/Yb(III).

Noteworthy, the PSCBS solution is a diluted solution. The coordination reaction among PSCBS and the Eu³⁺/Yb³⁺ ion occurs in an intramolecular action. It is suggested that the salicylaldehyde ligands hanging from the same macromolecule chain would coordinate to the Eu³⁺/Yb³⁺, and an intrachain complex would formed, as shown in Scheme 1 (2).

3.2 Analytical and physical data of the Schiff base polymer PSCBS

The contents of C, H, and N in the sample were determined by elemental analyses. The results were listed in Table 2. The results shows the content of C, H, and N measured in the compound not very different from the theoretical value. So the experimental values of the element analysis suggested that Schiff base polymer PSCBS have been synthetized.

3.3 Infrared spectrum of PSCBS and their complexes

In the IR spectrum of PSCBS (Figure 1), except for the characteristic bands of poly (styrene–co–butyl acrylate), there are some bands due to salicylaldehyde hydrazone moiety. The band at 1,619 cm^-1 is attributed to the stretching vibration absorption of –C=N– bond of the Schiff base, showing that the Schiff base reaction between poly(styrene–co–butyl acrylate) and salicylaldehyde hydrazone has occurred, indicating that salicylaldehyde hydrazone ligands have been bonded onto the side chains of poly(styrene–co–butyl acrylate). The stretching vibration absorption peak of the carbonyl group and the bending vibration absorption peak of phenolic hydroxyl group of the polymer functionalized with the Schiff base have appeared at 1,726 cm^-1 and 1,248 cm^-1, respectively.

Figure 1

Infrared spectra of PSCBS (a: P 1-0.5 b: P 1-1 c: P 1-1.5 d: P 1-2 e: P 1-2.5).

In the spectrum of PSCBS–Eu(III)/Yb(III) (Figure 2), compared to the infrared spectrum of PSCBS, the peak corresponding to the carbonyl group was transferred from 1,726 cm^-1 to 1,733 cm^-1, showing that the carboxyl group of BA has participated in the coordination to Eu³⁺/Yb³⁺ ion. The stretching vibration absorption of –C=N– bond of Schiff base has weakened greatly, indicating that the –C=N– bond of the Schiff base has participated in the coordination to Eu³⁺/Yb³⁺ ion. The bending vibration absorption of phenolic hydroxyl group at 1,248 cm^-1 has shifted to 1,270 cm^-1, whereas the absorption of phenolic hydroxyl group has weakened greatly, showing that the phenolic hydroxyl group of salicylaldehyde hydrazone has participated in the coordination to Eu³⁺/Yb³⁺ ion. So based on the facts that the ligand salicylaldehyde hydrazone on the side chains of PSCBS has coordinated to Eu³⁺/Yb³⁺ ion in a bidentate coordination form, and forming the polymer rare earth complex PSCBS-Eu(III)/Yb(III).

Figure 2

Infrared spectra of PSCBS–Eu(III)/Yb(III) (a: C 1-0.5 b: C 1-1 c: C 1-1.5 d: C 1-2 e: C 1-2.5).

3.4 UV absorption spectrum of PSCBS and their Eu(III)/Yb(III) complexes

Figure 3 shows the UV absorption spectra of the PSCBS. In the Figure 3, There are two absorption peaks at about

Figure 3

UV absorption spectrum of PSCBS (a: P 1-0.5 b: P 1-1 c: P 1-1.5 d: P 1-2 e: P 1-2.5).

294 nm and 354 nm. The former peak is primarily owed to the π*-π* electron transition of the benzene ring of polystyrene backbone. The latter peak is assigned to the n-π* electron transition of the keto and enol tautomerism of Schiff base ligands. The UV spectra of the polymer rare earth complexes (Figure 4) are similar to that of PSCBS in spectrum shape and the peak positions, and only the characteristic absorption intensities were reduced. The reduced absorption peak of the complexes imply that the Schiff base ligand of the polymer PSCBS has coordinated to Eu³⁺/Yb³⁺ and the polymer rare earth complex PSCBS-Eu(III)/Yb(III) has been formed.

Figure 4

UV absorption spectrum of PSCBS–Eu(III)/Yb(III) (a: C 1-0.5 b: C 1-1 c: C 1-1.5 d: C 1-2 e: C 1-2.5).

3.5 Thermal analysis of PSCBS and their Eu(III)/Yb(III) complexes

The thermal analysis curves of the PSCBS was depicted in Figure 5. In the present study, the weight loss was

Figure 5

Thermal spectra of PSCBS (a: P 1-0.5 b: P 1-1 c: P 1-1.5 d: P 1-2 e: P 1-2.5).

measured from the ambient temperature up to 800°C. From TGA curves we can see that, all samples showing good thermal stability and show two main degradation steps. A slight weight loss of five samples happened at the temperatures over 160°C because of desorption of physically adsorbed water and residual solvent THF. The 20% weight loss took place in the temperature range of 180°C ~350°C may be attributed to the loss of some aromatic fragments, without any decomposition of the chemical bond. With the increase of styrene molar ratio, the amount of weight loss increased gradually. On further heating, the TGA curve exhibited decomposition of the organic molecular chains of PSCBS till a complete weight loss, which is accompanied by an endothermic process in the DSC curves (not given).

Differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA) are performed on all the complexes. Figure 6 shows the TGA traces of complex C1-0.5, C1-1, C1-1.5, C1-2 and C1-2.5. From the TGA curves we can see that all samples show similar weight loss trend

Figure 6

Thermal spectra of PSCBS–Eu(III)/Yb(III) (a: C 1-0.5 b: C 1-1 c: C 1-1.5 d: C 1-2 e: C 1-2.5).

in, and two main degradation steps can be obviously observed. The initial weight loss occurring in the temperature range 200~330°C is interpreted as loss of crystal water molecules and coordinated water molecules for PSCBS–Eu(III)/Yb(III) complexes. Degradation of the ligand was observed in the range of 330~430°C. At 430, the complex completely converts into Eu₂O₃/Yb₂O₃, and the weight becomes stable. Compared with the organic compound PSCBS, whose melting point is only 80~110°C, thermal stabilities of this kind of complexes are largely improved. This improvement of thermal stability is interpreted by the formation of the M–O bond.

3.6 Fluorescence emission intensity dependence on salicylaldehyde hydrazine ligand amount for the complex PSCBS–Eu(III)/Yb(III)

The fluorescence excitation spectrum of PSCBS– Eu(III)/Yb(III) was obtained by monitoring the emission of Eu³⁺/Yb³⁺ at 497 nm, and the peak at 433 nm was found to be the optimal excitation peak. By exciting at 433 nm, the fluorescence emission spectrum of PSCBS–Eu(III)/Yb(III) in DMF solution was decided. By fixing the concentration of Eu³⁺/Yb³⁺ in the solution, the concentration of salicylaldehyde hydrazone ligand was changed gradually and the molar ratio of the butyl acrylate to styrene changed to (1:0.5; 1:1; 1:1.5; 1:2; 1:2.5), and the fluorescence emissions of the complex PSCBS–Eu(III)/Yb(III) in these solutions were decided. The results are displayed in Figure 7.

Figure 7

Fluorescence emission spectrum of PSCBS–Eu(III)/Yb(III) (a: C1-0.5 b: C1-1 c: C1-1.5 d: C1-2 e: C1-2.5).

In Figure 7, the following results are showed: (1) the fluorescence emission of the complex PSCBS–Eu(III)/Yb(III) (e) is very weak and the concentration of salicylaldehyde hydrazone in the solution is small as seen in the analytical and physical data of the Schiff base polymer PSCBS (Table 1). (2) the fluorescence emission of the complex PSCBS–Eu(III)/Yb(III) increased the molar ratio of butyl acrylate to styrene, and reached a maximum value in the molar ratio of 1:1. The maximum luminescent intensity was observed at 489 nm, and the half width maximum was about 100 nm. Therefore, the fluorescence emission of the PSCBS–Eu(III)/Yb(III) was mainly sensitized by the absorption of the salicylaldehyde hydrazone group on the side chains of macromolecule ligand PSCBS.